Infectious diseases are illness in plants or animals caused by pathogenic microbial agents, including pathogenic viruses, bacteria, fungi, protozoa, and multicellular parasites, and aberrant proteins known as prions. These illnesses are often communicable because of transmission of the infectious replicating pathogenic agent from one person or species to another. Transmission of an infectious disease may occur through direct physical contact with infected individuals, through liquids, food, body fluids, contaminated objects, airborne inhalation, or vector-borne (e.g., insects etc.) distribution.
Despite the fact that infectious diseases, including Tuberculosis (TB), are the third-leading cause of death in the U.S. and the second-leading cause of death world wide, the FDA approval of new antibacterial agents decreased by 56% during the last 20 years. Between 1998 and 2004, only 9 new antimicrobials were approved by the FDA, and only two of these represented new antimicrobial mechanisms. In addition, a survey of the world's 15 largest pharmaceutical companies revealed that only 30 of 315 New Molecular Entity Drugs (NMEs) were identified as anti-infective agents, and only five of these were new antibacterial agents. None of the five represented novel mechanisms of action. The development of antimicrobial drugs for TB is even more dismal. After the introduction of rifampicin, no worthwhile anti-TB drug with a new mechanism of action has been developed in more than 30 years.
According to the WHO, there were 9.3 million incident cases of TB and 13.7 million prevalent cases of TB in 2007 (WHO, 2009). There also were 1.3 million deaths from TB among HIV-negative people in 2007, and an additional 450,000 deaths among HIV-positive TB cases—equivalent to 23% of deaths attributable to HIV. This equates to one death from TB every 20 seconds, which could be alleviated by effective antibiotic treatment. Because 86% of all cases occur in Africa and Asia, it is likely that many cases of TB go undiagnosed and/or untreated. Similar statistics are available for other pathogenic microbial agents including viruses, fungi, protozoans, and prions.
The development of drug resistance in pathogenic microorganisms has increased the urgency of the need to develop new antimicrobial compounds. The CDC recently reported that in American hospitals alone, healthcare-associated infections account for an estimated 1.7 million infections and 99,000 associated deaths each year. Of these infections 32 percent of all healthcare-associated infection are urinary tract infections, 22 percent are surgical site infections, 15 percent are pneumonia (lung infections), and 14 percent are bloodstream infections. Methicillin-resistant Staphylococcus aureus (MRSA), for example, is a type of staph bacteria that is resistant to beta-lactam antibiotics. These antibiotics include methicillin and other more common antibiotics such as oxacillin, penicillin, and amoxicillin. Although most MRSA infections are skin infections, the infections can spread to the blood stream, lungs, heart, bones, and joints, and they can be fatal because of the ineffectiveness of currently available antibiotics against the pathogen. Other common drug resistant bacteria include Klebsiella pneumonia, Acinetobacter, Streptococcus pneumonia, Drug-resistant TB (Tuberculosis), and Vancomycin-resistant Enterococci (VRE). Drug resistance also can arise in pathogenic fungi, protozoans, and viruses, thereby increasing the risk and severity of infections caused by these microbial agents.